The present invention relates to methods of inspecting the finger dovetails of a turbine wheel and buckets for cracks in the material surrounding pinholes in which pins are received for securing the buckets and wheel to one another and particularly relates to use of a phased array ultrasonic probe for finger dovetail inspection in situ for cracks in the fingers.
In turbines, for example, steam turbines, the rims of the turbine wheels are often provided with axially spaced, annular extending fingers defining dovetails which receive generally complementary-shaped discrete finger dovetails on buckets secured to the wheel. The bucket and wheel dovetails interdigitate with one another and at least two, and typically three, pinholes are aligned axially through bucket and wheel fingers along the margin of the wheel with the pinholes lying along a radius at each bucket location. Pins are axially inserted through the aligned pinholes to maintain the buckets secured to the wheel. It will be appreciated that the pins bear the radial loading of the buckets on the wheel. Over time and extended use, the radial loading may cause stress-related cracks to develop at or in the general region of one or more of the wheel fingers, particularly at the pinholes in the turbine wheel fingers. The stress-related cracks tend to have a generally tangential orientation and typically propagate circumferentially from the pinholes. The cracks sometimes link up with adjacent pinholes. It will be appreciated that crack formation in either one or both of the wheel and bucket finger dovetails provides a potential for failure of the wheel or bucket dovetail, loss of the bucket at speed and damage to the turbine and/or the power station.
This potential for turbine failure has been recognized in the industry. Consequently, periodic inspections of the wheel and bucket finger dovetails are indicated. Periodic inspections can, of course, be performed by disassembly of the buckets from the turbine wheel. However, to disassemble each bucket or even sample buckets from the turbine wheel after usage of the turbine is labor-intensive, time-consuming and, hence, costly. Additionally, the pins securing the buckets to the turbine wheel are oftentimes extremely difficult to remove to release the bucket from the wheel. Typically, the pins are hammered out or a gun with an explosive charge is used to dislodge those particularly hard-to-remove pins. Also, drilling and EDM processes have been used for pin removal. Upon removal of the pins and buckets from the wheel, the finger dovetails may be inspected, e.g., by using magnetic particle testing techniques. After testing, the buckets and pins must be reinstalled in the wheel. Some of the pins, however, may have been damaged upon their removal and must be replaced. Also, magnetic particle inspection requires surface preparation prior to inspection. Further, significant logistic support in the form of cranes, laydown areas and the like is required in order to complete removal, inspection and reinstallation of the pins to secure the buckets to the wheel. Accordingly, there has developed a need for non-destructive in situ inspection of turbine wheel and bucket finger dovetails.
Accordingly, and in a preferred embodiment of the present invention, there is provided a method of inspecting in situ material adjacent the pinholes in the finger dovetails of the wheel and buckets for crack formation with removal of only a minimum number of pins and without removing the buckets from the wheel. Thus, pins are removed from the pinholes at preferably regularly spaced intervals about the wheel, e.g., every other bucket, and then only one and preferably the intermediate pin of the three pins securing the bucket to the wheel. Upon removal of the selected pins, an ultrasonic probe, with either one or more discrete elements, or alternately a phased array ultrasonic probe is then inserted into the pinholes to detect crack formation in both circumferentially and radially adjacent pinholes. In the case of an ultrasonic probe with one or more elements, the probe is mechanically rotated in and about the axes of the aligned pinholes, thereby providing a full circumferential scan of the material about the aligned pinholes. The ultrasonic information is then analyzed for detection of cracks.
In the case of the phased array ultrasonic probe, the probe is sized for insertion into the pinholes whereby the ultrasonic beam can be electronically scanned circumferentially without mechanical movement of the probe, i.e., without rotation of the probe about the axis of the pinhole. The ultrasonic beam can also be focused at different distances. The position of the probe can be encoded and combined with the ultrasonic information, enabling accurate imaging of the inspection data for analysis. The combination of mechanically moving the ultrasonic probe in the axial direction and rotating the ultrasonic beam circumferentially by pulsing individual phased array elements, e.g., piezoelectric elements, about the circumference of the probe with appropriate delays, permits a complete scan of the material about the pinhole. By synchronizing the axial scan with the ultrasonic pulsing, a continuous helical scan path can be produced. Alternatively, a circumferential scan can be conducted at one axial distance and the probe can then be incrementally and repeatedly axially advanced to create a plurality of scan/indexing steps to inspect the material adjacent the axial length of the pinhole. It will be appreciated that the ultrasonic beam thus detects crack formations opening into holes adjacent to the pinhole receiving the probe. Crack formation in the pinhole receiving the probe can be ascertained by another testing technique, such as eddy current tests, after withdrawal of the probe. As a consequence of the foregoing, the inspection process can be performed in situ with a minimum number of pins removed from the pinholes and without removal of buckets from the turbine wheel, while still obtaining a high sensitivity to crack detection in the finger dovetails of the wheel and buckets.
As a further technique, the inspection method hereof may involve a sampling of the finger dovetails at intervals about the wheel insufficient to detect all cracks. That is, the ultrasonic sampling probe may be used only in widely-spaced pinholes and therefore not lie in position to detect all cracks. In this manner, a statistical probability of crack formation can be determined, e.g., non-existent, very low, high probability or the like. Once the probe detects any cracks extant in the dovetail material, the cracks can be further investigated, e.g., by removing the bucket and performing other tests to determine the extent of the crack. Also, the inspection interval about the wheel can be reduced to ensure detection of all cracks, rather than just a sampling.
In a preferred embodiment according to the present invention, there is provided a method of inspecting finger dovetails of at least one of a turbine wheel and bucket, the wheel bucket having aligned holes through the finger dovetails for pinning the wheel and bucket to one another, comprising the steps of inserting an ultrasonic probe in a hole of the aligned holes in one of the wheel and the bucket and rotating the probe within and about axes of the aligned holes to electronically scan material about the hole to identify any cracks extant in the material about the hole.
In a further preferred embodiment according to the present invention, there is provided a method of inspecting finger dovetails of at least one of a turbine wheel and bucket, the wheel bucket having aligned holes through the finger dovetails for pinning the wheel and bucket to one another, comprising the steps of inserting a phased array ultrasonic probe in a hole of the aligned holes in one of the wheel and the bucket and actuating the probe to electronically scan material of a finger dovetail of the one wheel and bucket circumferentially about the hole to identify any cracks extant in the material about the hole.
In a further preferred embodiment according to the present invention, there is provided a method of inspecting in situ turbine wheel and bucket dovetails for cracks in materials about pinholes containing pins securing the buckets and wheel to one another, comprising the steps of (a) removing a pin from a pinhole securing one of the buckets to the wheel, (b) inserting a phased array ultrasonic probe into the pinhole and (c) actuating the probe to electronically scan material circumferentially about the pinhole to identify cracks extant in the material about pinholes adjacent to the pinhole receiving the phased array ultrasonic probe.